1. Field of the Invention
This invention relates to an electronic optical system, and more particularly to a light projection system suitable for a use in a liquid crystal display system (LCD) to display image.
2. Description of Related Art
Recently, a LCD device is widely used in TV, computer, monitor or other display system. Comparing with a conventional display apparatus with a picture tube, the LCD system is lighter and has a smaller dimension. It becomes a necessary part to display system, such as a notebook computer.
FIG. 1 is a structure diagram, illustrating a polarization light projection system used in a reflection-type color display board as disclosed by U.S. Pat. No. 5,530,489. In FIG. 1, a reading light source 100 can emit a white light. The white light is polarized by a polarization beam splitter (PBS) 102 and split into an S-state polarized beam and a P-state polarized beam, both which are also reflected so that both light polarized beams are deflected by 90 degrees. The S-state polarized beam forms the WS beam. The P-state polarized beam is converted into an S-polarization beam WS' through a half-wave plate 106. The WS and WS' beams are incident to a polarization analyzer 108 and are deflected by 90 degrees again, in which the polarization analyzer 108 further ensures that the Ws and the WS' beams are polarized into an S polarized state.
The WS and the WS' beams enter a dichroic prism 110, which deflects a blue light BS of the WS and WS' beams by 90.degree. and allows a red light RS and a green light GS to continuously transmit. The blue light BS passes a light path compensation plate 112 and enters a blue liquid crystal light valve (LCLV) 114, which converts the blue light BS into a blue light BP with P-state polarization and reflect the blue light BP back to the dichroic prism 110 through the light path compensation plate 112. The blue light BP is deflected to a projection lens 122 through the polarization analyzer 108. The projection lens 122 project the blue light BP onto an image screen (not shown). For the red light RS and the green light GS, they continuously travel to a color filter prism 116, which deflects the green light GS by 90.degree. into a red LCLV 118, and allows the red light RS to pass and reach a green LCLV 120. The red LCLV 118 reflects the green light GS back and converts it into a green light GP with P-state polarization. Similarly, the green LCLV 120 reflects the red light GS back and converts it into a red light GP with P-state polarization. The red light RP and the green light GP are also deflected to the projection lens 122 and projected to the image screen like the blue light BG.
In this system shown in FIG. 1, the system includes two light splitters and several prisms, resulting in a large system dimension. This display system cannot be efficiently applied in a large displaying area and is not portable. Moreover, a poor focusing quality severely occurs due to a too large distance between the LCLVs and the projection lens. This further limits its applications.
Another system is disclosed by U.S. Pat. No. 5,153,752 to reduce the distance of the projection lens and the system dimension. FIG. 2 is a structure diagram, illustrating a polarization light projection system used in a reflection-type color display board as disclosed by U.S. Pat. No. 5,153,752. In FIG. 2, a light source 200 can emit a white unpolarized light S+P, which enters a PBS 201 and is split into an S-state polarized beam S1 and a P-state polarized beam P2. The S-state polarized beam S1 is deflected by 90.degree. and enters a dichroic prism set 204, which includes several dichroic prisms 204a, 204b, 204c, and 204d. After passing the dichroic prism set 204, the S-state polarized beam S1 are split into a red light RS, a green light GS, and a blue light BS, which respectively travel to LCD panels 205R, 205G and 205B. The LCD panels 205R, 205G and 205B respectively convert the red light RS, the green light GS, and the blue light BS into a red light RP, a green light GP, and a blue light BP with P-state polarization, and reflect the lights RP, GP, and BP onto a projection lens 206, which projects passing light onto a screen (not shown).
For the P-state polarized beam P2, as it passes the PBS 201, it enters a half-wave plate 202 and is polarized to an S-state polarized beam S2. The S-state polarized beam S2, similar to the S-state polarized beam S1, is reflected by the LCD panels 205R, 205G and 205G and reach the screen at the end.
In this conventional projection system of FIG. 2, the dimension and the light focusing issues of the conventional projection system of FIG. 1 is reduced. However, since the system of FIG. 2 is very complicate, production yield rate is low and fabrication cost is high. Moreover, since several prisms are used in the system, a little misalignment may cause a large error. Its requirement of alignment precision is much higher that a usual level.